This invention relates to an improvement of a surface analysis method and particularly to a surface analysis method and apparatus suitable for chemical state analysis of small areas on the sample surface to be analyzed.
The development in small surface area analysis is remarkable, and this small area analysis and its technology are generally called "micro-characterization".
The information to be obtained from the small surface area includes the geometrical shape and structure of the surface and the, nearby surface region (hereinafter called the surface), the atomic species, composition and chemical bonds of these atoms within the region, and so on. In addition, the analysis must be non-destructive.
Of the above-given information, the geometrical shape of the surface can now be measured by an STM (scanning tunnelling microscope) and a high resolution SEM (scanning electron microscope). At the present time, the lateral resolution with which the shape is measured is close to 1 .ANG..
On the other hand, the lateral resolution in analyzing the chemical state such as atomic species and chemical bonds (that is, the extent of the small surface area from which the atomic species and chemical bonds can be identified) is now several tens of .mu.m which is far poorer as compared with that in measuring the shape.
However, high lateral resolution in analyzing the chemical state seems to be required in the future for the following reasons. For example, the residues on semiconductor circuit devices after surface treatment will remarkably deteriorate the device characteristics. In order to analyze and identify the residues on the actual sample surface, it is necessary to increase the lateral resolution to the minimum pattern size of the devices. When the device pattern size of 0.3 to 0.1 .mu.m is considered to be achieved in the future, the lateral resolution of 0.1 .mu.m or less will be required in small area analysis.
Moreover, the actual surface has many defects such as steps. The deposition of materials and chemical reaction such as catalytic reaction on the surface will not advance uniformly due to the presence of the defects. This non-uniformity seems to be in the order of 0.1 to several .mu.m from the observation of grain boundary (Katsumichi Yagi, et al., Oyo-butsuri, 55, 1036-1050 (1986)). From the standpoint of analyzing the nonuniform phenomena, the resolution of 1 .mu.m or less will also be required.
In the analysis of single protein molecules which is requested in the biology and biological engineering, the lateral resolution of 0.01 .mu.m or less is necessary because the protein molecule size is several tens of .ANG..
The following methods have been proposed for improving the lateral resolution in the surface analysis for obtaining chemical state information.
One of the methods is focusing soft x-rays by a single crystal surface with a curvature in XPS (X-ray photoelectron spectroscopy) (F. J. Grunthaner, MRS Bulletin 30, 60-64 (1987)). In this method, however, the aberration of the optical system is very large, so the beam is limited to about 120 .mu.m.phi.. Therefore, the size of the area to be analyzed is 120 .mu.m.phi., so that the lateral resolution of 1 .mu.m or less cannot be achieved by the method.
Another method for improving the lateral resolution is to generate a strong magnetic field near the sample surface, and catch the photoelectrons emitted from the sample surface by using electron cyclotron motion. The drawback of this method is the fact that the lateral resolution is determined by the Larmor radius r.sub.b of the photoelectron: EQU r.sub.b =v.sub.p m/(eB) (1)
where v.sub.p, m, e, and B are the velocity component of photoelectron measured perpendicular to the magnetic field, the mass of electron, the charge of electron, and the magnetic flux density near the sample surface, respectively. Assuming that the photoelectron is emitted with a kinetic energy E at an angle .theta. (which is measured from the magnetic field direction) eq. (1) is rewritten in terms of E and .theta. as EQU r.sub.b =.sqroot.2Em sin .theta./(eB) (2)
The lateral resolution by this method is actually determined by 4 r.sub.b. As an example, substituting E=10 eV B=20T, .theta.=90.degree. into eq. (2) will yield 4 r.sub.b =2.1 .mu.m. Thus, the lateral resolution in this method is normally on the order of .mu.m.
In order to achieve a higher resolution, the angle .theta. should be small as is indicated from eq. (2). This means that only a small fraction of the photoelectrons emitted from the sample surface can be utilized in the surface analysis. For example, considering a photoemission differential cross section (R. F. Reilman et al., J. Elect. Spectrosc. Relat. Phenom. 8, 389-394 (1976)), the utilization efficiency of photoelectrons is 1% or less when the lateral revolution is set to 0.06 .mu.m (4 r.sub.b). In addition, the utilization efficiency is more decreased with further increasing lateral resolution. Therefore, this method has a serious disadvantage that photoelectrons can hardly be observed although they are emitted from the sample surface. Moreover, since the lateral resolution depends on kinetic energy E (eq. (2)), the photoelectron kinetic energy cannot be analyzed with a constant lateral resolution.
There is another method that an x-ray is focused by use of a Fresnel zone plate (Japanese Patent Laid-Open Number 265555/1987). This method, however, has a low utilization efficiency of the x-ray. For example, even for the most intense first-order diffraction, the theoretical value of the efficiency is about 10%, while the measured value is about 5%.
An abundant x-ray flux is required for analyzing a small surface area in order to obtain a large detection signal intensity. This is because the number of atoms and molecules in the area is very small. Focusing an x-ray only by the zone plate brings about low signal intensity which is a serious problem of the analysis.
As described above, the conventional methods have a disadvantage of low lateral resolution or low signal intensity. Therefore, it is impossible to obtain chemical state information with lateral resolution of 1 .mu.m or less.